Polyolefins particularly such as polypropylene are excellent in heat-sealing properties, mechanical strength and water resistance, lightweight, nontoxic, tasteless and odorless and, moreover, have heat resisting properties capable of withstanding the retort treatment. Utilizing such properties as mentioned above, polyolefins particularly such as polypropylene are widely used as packaging materials such as food packaging films, cups, bottles, etc. However, polyolefins such as polypropylene had such problems that because they are relatively high in permeability to gases such as oxygen, carbon dioxide gas, etc. and are somewhat poor in oil resistance and grease resistance, they cannot be used alone in packaging perishable food such as fish meat, edible meat or processed products thereof, food rich in fats and oils, and the like food.
In order to solve such problems as mentioned above, there has been proposed to laminate polyolefins particularly such as polypropylene with polyvinylidene chlorides or saponification products of ethylene/vinyl acetate copolymer, both of which are of excellent gas barrier and oil resistance properties. Particularly, polyvinylidene chlorides are suitable as materials for gas barrier layers of food packaging containers which require the retort treatment, because polyvinylidene chlorides are low in hydroscopicity and exhibit no decrease in gas barrier properties when used as the gas barrier layers. However, the existing circumstances were such that because polyolefins such as polypropylene have no polar groups, even when a polyolefin layer is directly laminated with a layer of polyvinylidene chloride or saponification product of ethylene/vinyl acetate copolymer, the resulting lamination layers are very low in adhesion therebetween and the laminate thus obtained is of little practical use.
As procedures for improving adhesion between a polypropylene layer and a polyvinylidene chloride layer, there have heretofore been proposed a process in which a modified polypropylene grafted with an unsaturated carboxylic acid or its derivative is used for forming a polypropylene layer (Japanese Patent Publin. No. 14970/1982) and a process in which a resin comprising a mixture of a low crystalline ethylene/.alpha.-olefin copolymer and an ethylene/vinyl acetate copolymer, at least one of which has been graft-modified with an unsaturated carboxylic acid or its derivative, is used form an adhesive layer to be interposed between the polypropylene layer and polyvinylidene chloride layer (Japanese Patent L-O-P Publin. No. 13718/1980).
In these processes as referred to above, adhesion between the layers after the retort treatment or such high temperature treatment as high temperature filling markedly decreases, though the initial adhesion between the layers is high.
Further, polyethylene terephthalate resin is excellent in mechanical strength, rigidity, heat resistance, chemical resistance, oil resistance, transparency, etc., and widely used as packaging materials such as films, sheets, containers, etc., by utilizing these characteristics as mentioned above. However, because of its not so low permeability to gases such as oxygen, etc., such it could not be used as packaging materials for products such as food, chemicals, cosmetics and the like for which high gas barrier properties of the packaging materials are strictly required.
Furthermore, because of its not so lows permeability to gases such as oxygen, etc., polycarbonate resins could not be used as packaging materials particularly for such products as food and the like for which high gas barrier properties of the packaging materials are strictly required, though the polycarbonate resins are excellent in transparency, heat resistance, perfume barrier properties, etc., similarly to the polyester resins.
In order to improve gas barrier properties of the polyethylene terephthalate resins or polycarbonate resins, there has been proposed a procedure wherein the polyethylene terephthalate resins or polycarbonate resins are laminated with a resin which is superior in gas barrier properties to these resins, for example, saponification products of ethylene/vinyl acetate copolymers. In the proposed procedure, however, adhesion between the polyethylene terephthalate or polycarbonate resins and the ethylene/vinyl acetate copolymer saponification products is not always sufficient, and in some cases parts of the lamination layers peel off from each other at the time of lamination or use thereof, gas barrier properties of the laminate obtained decreases, or the lamination products prepared therefrom are marred in appearance or deteriorate in mechanical strength.
Japanese Patent L-O-P Publin. Nos. 270155/1986 and 158043/1987 disclose laminates which are obtained by joining polycarbonate or polyester layers to layers of saponification products of ethylene/vinyl acetate copolymers by the use as an intermediate adhesive layer of graft-modified ethylene/.alpha.-olefin copolymers graft-modified with unsaturated carboxylic acids or derivatives thereof. The disclosed laminates have excellent gas barrier properties and the layers of the laminate adhere strongly each other at ordinary temperature, However, when these laminates are subjected to heat treatment such as high temperature filling or retort treatment, adhesive strength between layers of the laminates sometimes decreases after such treatments, and on that account those layers peel off from each other and gas barrier properties of the laminates decreases.